1. Field of the Invention
The invention is directed to an improved fuel injection system for an internal combustion engine.
2. Description of the Prior Art
A fuel injection system of the type with which this invention is concerned is known from DE 101 32 732 A and has a respective high-pressure fuel pump and a fuel injection valve connected to it for each cylinder of the internal combustion engine. The high-pressure fuel pump has a pump piston that is driven into a stroke motion and delimits a pump working chamber. The pump working chamber can be connected to a low-pressure region via a connection controlled by a first electrically actuated control valve. The fuel injection valve has an injection valve element that controls at least one injection opening and that is acted on in an opening direction by the pressure prevailing in a pressure chamber connected to the pump working chamber. An electrical control element controls an opening and closing motion of the injection valve element. The fuel injection valve here has a control pressure chamber that can be connected to the pump working chamber and can also be connected to a relief region via a connection controlled by the control element, which is embodied as a second electrically actuated control valve. A pressure reservoir is also provided into which fuel is delivered by the high-pressure fuel pump and which is connected to the pressure chamber of the fuel injection valve. Fuel can be drawn from the pressure reservoir for an injection, independent of the delivery by the high-pressure fuel pump. In particular, this permits a secondary injection of high-pressure fuel, which can occur at a time when the high-pressure fuel pump has already stopped delivering fuel. A secondary injection of this kind is advantageous for reducing emissions of the engine, especially particulate emissions. The connection of the pump working chamber and pressure chamber to the pressure reservoir contains a throttle restriction and, parallel to this, a check valve that opens toward the pressure chamber. A filling of the pressure reservoir with fuel occurs only via the throttle restriction, which must be large enough to permit a sufficient filling of the pressure reservoir even when the pressure generated by the high-pressure fuel pump is not very high and when the fuel injection quantity is low. In addition, after the termination of fuel injection, a high pressure must be maintained in the pressure chamber in order to be able to deliver a large fuel quantity into the pressure reservoir, which requires a large amount of driving work from the high-pressure fuel pump, thus resulting in a poor efficiency of the fuel injection system. Because of the significant pressure differences between the pressure reservoir on the one hand and the pump working chamber and pressure-relieved pressure chamber on the other, an expensively designed check valve is required in order to assure a reliable seal between them.